See screenshot. There are two different factors: the real difference in size (height x width) and the optical difference in size. The latter factor is the most important. "Tabs", "Privacy", "Security", "Advanced" match together in optical size. "Main" matches in style but the real size is somewhat too big. The two icons that look inconsistent are "Content" and "Feeds" "Feeds" looks a lot bigger because the real size is bigger but also because of the optical size (it is more massive, there is less background space and the color is a lot more prominent than e.g. "Advanced"). "Content" is bigger than "Advanced" in real size but also the color saturation is too heigh. I would suggest this: "Privacy", "Security" an "Tabs" should stay the same. "Advanced" (which has a grey metal color) should get another color, e.g. light purple or blue (this to match better with "Content" and "Feeds"). "Main" should be one or two pixels smaller. "Content" should also be a bit smaller + less saturation in color. "Feeds" should be one or two pixels smaller + less saturation in color (more grey factor).
I don't mean to offend in any way, but what's wrong with the icons having partially different sizes, mostly 1-2 pixels?
(In reply to comment #2) > Like already described, the way the eye experiences size and distance of an object depends on different factors indeed.
This looks even worse on Mac. Surely we can set a min-width on the Options tabs that gets this closer to right at least for en-US.
--> blocking to make a decision on this one way or another
For Mac, this is the way pref panels work; go take a look at Mail.app and check out the space between Junk Mail and Fonts & Colors. For Windows, this is the same as we had in Fx1.5, and I'm not sure what fixing it to add padding between the icons will do. fwiw, the icons are all in 32x32 blocks, but they don't always go edge-to-edge. They are centered within their regions. Marking WONTFIX.
Marking fixed1.8.1 to match resolution to remove this bug from the blocking radar.